Glass ceramic and process therefor

ABSTRACT

1. A PROCESS OF MAKING AN ARTICLE OF GLASS-CERAMIC WHICH COMPRISES (1) FORMING AN ARTICLE FROM A THERMALLY CRYSTALLIZABLE GLASS CONTAINING AT LEAST ONE INGREDIENT THAT IS LITHIA THAT COMBINES WITH ALUMINA AND SILICA TO PROVIDE BULK CRYSTALLIZATION BY A HEAT TREATMENT, A NUCLEANT PRESENT IN SUFFICIENT CONCENTRATION FOR THE BULK CRYSTALLIZATION, AND SODIUM OXIDE IN A CONCENTRATION THAT INHIBITS THE RATE OF SAID BULK CRYSTALLIZATION WITHOUT PREVENTING SURFACE CRYSTALLIZATION OF SAID GLASS, (2) HEAT TREATING THE ARTICLE OF GLASS AT AN ELEVATED TEMPERATURE SUFFICIENT TO PROVIDE CRYSTALLIZATION OF A SURFACE LAYER OF THE ARTICLES WHILE THE MAIN BODY PORTION REMAINS A GLASS, AND (3) HEAT TREATING THE SURFACE CRYSTALLIZED ARTICLE AT A HIGHER ELEVATED TEMPERATURE TO CONVERT THE MAIN BODY PORTION BY BULK CRYSTALLIZATION TO A GLASSCERAMIC, SAID LITHIA BEING PRESENT IN A SUFFICIENT CONCENTRATION IN THE THERMALLY CRYSTALLIZATION GLASS TO PROVIDE IN ABSENCE OF SAID RATE-INHIBITING SODIUM OXIDE, BULK CRYSTALLIZATION UNIFORMLY THROUGHOUT THE ARTICLE, AND SAID SURFACE LAYER HAVING A LOWER COEFFICIENT OF LINEAR EXPANSION THAN THAT OF THE MAIN BODY PORTION OF THE ARTICLE OF GLASS-CERAMIC, AND THEREAFTER (4) HEATING A SURFACE LAYER OF THE ARTICLE TO AN ELEVATED TEMPERATURE SUFFICIENTLY HIGH TO CONVERT THE GLASSCERAMIC OF TH SURFACE LAYER TO GLASS WHILE MAINTAINING THE MAIN BODY OF THE ARTICLE AS GLASS-CERAMIC, (5) ION-EXCHANGE THE SURFACE LAYER AFTER ITS CONVERSION TO GLASS TO REPLACE AT LEAST PART OF THE SODIUM IONS BY LITHIUM IONS, AND (6) HEAT TREATING THE ARTICLE TO CONVERT THE GLASS OF THE SURFACE LAYER TO GLASS-CERAMIC CONTAINING BETAEUCRYPTITE AS PREDOMINANT CRYSTAL WHEREBY SAID SURFACE LAYER HAS A LOWER EXPANSION COEFFICIENT THAN THAT OF THE MAIN BODY PORTION.

United States Patent 3,843,342 GLASS CERAMIC AND PROCESS THEREFOR GeorgeA. Simmons, Toledo, Ohio, assignor to Owens-Illinois, Inc.

No Drawing. Original application June 17, 1966, Ser. No. 558,238, nowabandoned. Divided and this application Aug. 17, 1971, Ser. No. 144,315The portion of the term of the patent subsequent to Mar. 3, 1987, hasbeen disclaimed Int. Cl. C03c 21/00; (3031: 31/00 US. CI. 65-30 1 ClaimABSTRACT OF THE DISCLOSURE This invention relates generally to articlesof glassceramic, including glass-ceramic components of articles, and toprocesses for manufacturing these glass-ceramic articles. The process ofthis invention replaces sodium ions in the surface of an article withlithium, after the article is subjected to crystallization andvitrification steps. The glass article is first surface crystallized toform a crystal skeleton, after which the bulk of the glass article iscrystallized. Then the surface is vitrified by flame treatment so thatlithium can replace sodium in the surface. Finally, the glass surface isonce again heat treated to develop crystals.

The present application is a divisional of application Ser. No. 558,238,filed June 17, 1966, now abandoned.

More particularly, the present invention relates to a glass-ceramicarticle having a main body portion and at least one integral surfacelayer that differs from the body portion in some manner. The main bodyportion of the article is a glass-ceramic including at least 1% byweight, expressed as soda mole equivalent, of an oxide of at least onealkali metal, i.e., including an alkali metal oxide in a concentrationthat, if not soda, its replacement by an equivalent molar amount of sodawould provide a composition containing at least 1% by Weight of soda,based only on the substituted soda. In other words, any soda alreadypresent in the composition is not considered as being soda in thecomputation of the 1% limitation based on the alkali metal molarequivalent substitution. At least 2% by weight is preferred.

That different surface layer of the article is either a glass or aglass-ceramic that (1) has an overall composition in which (a) thelithia content when the layer is a glass-ceramic is at least equal toand when the layer is a glass is greater than the lithia content in theoverall composition of the main body portion and (b) the content ofoxide of alkali metal, having an ionic radius larger than lithium, whenthe layer is a glass-ceramic is at a maximum equal to and when the layeris a glass is less than the content of that oxide in the overallcomposition of the glassceramic main body portion, and 2) has a lowercoefficient of linear expansion than that of the main body portion. Thearticle has a compressive stress in that surface layer and a tensilestress in the main body portion.

The surface layer has a composition in which the mole percentageconcentration of any alkaline earth metal oxide is usually equal to themole percentage concentration of the same alkaline earth metal oxide, ifpresent, in the main body portion of the article. Some loss of alkalineearth metal oxide in the surface layer may occur during the making ofthe article in the event a high temperature is used in some cases ofheat treatment, but its content remains substantially unchanged.

In accordance with this invention, the article of the invention can bemade by carrying out one of the processes of the invention. Some of theprocesses are alternatives that can use an article of the same initialglass, whereas the use of these alternative processes or other processesis 'ice dependent upon the nature of the initial glass of the article tobe treated for the manufacture of the article of the present invention.

In the article of the invention in which both the main body portion andthat surface layer are glass-ceramics, either the crystalline phase ofthe surface layer differs from that of the main body portion or theglass matrix of the surface layer differs in lithia content from that ofthe main body portion. Both differences may be present. When thedifference is the lithia content of the glass matrix, the predominantcrystal in the crystalline phase of the surface layer may have eitherthe same as or a greater lithia content than that of the predominantcrystal of the crystalline phase of the main body portion.

In carrying out all embodiments, except one, of the process of thepresent invention there is provided at some stage of the process anion-exchange treatment in which the lithia content of the surface layeris increased without a comparable increase of the lithia content of themain body portion of the initial or intermediate article. In theion-exchange step at least part of the content of an alkali metal ionhaving a larger ionic radius is replaced in the surface layer by lithiumions to obtain the product of this invention. Usually the processrequires steps after the ion exchange but in one embodiment it is thelast important step. The alkali metal ion having the larger ionic radiusthat is replaced by lithium ion is preferably sodium ion.

An illustrative embodiment of the product of the invention has as theglass-ceramic of the main body portion of the article a crystallinephase in which beta-spodumene predominates and as the surface layer aglass-ceramic in which (1) the same crystal predominates in thecrystalline phase but the glass matrix of the surface layer has a higherlithia content or (2) the surface layer has betaeucryptite crystalspredominant in the crystalline phase regardless of any difference in theglass matrix. In either event the surface layer of the article has alower average coefiicient of linear expansion than that of the main bodyportion of the article and as a result the surface layer has acompressive stress and the main body portion has a tensile stress toprovide a greater flexural strength than that of an article entirely ofthe same glass-ceramic as that main body portion.

In another illustrative embodiment, the main body portion is aglass-ceramic, that has an average coeflicient of linear expansionsubstantially higher than glass-ceramic in which beta-spodumene is thepredominant crystal in the crystalline phase, and the surface layer is(1) a glass-ceramic with the same type of crystalline phase as the mainbody portion but with a greater concentration of crystalline phase orwith a glass matrix that has a lower linear coefficient of expansionthan the glass matrix of the main body portion, or (2) a glass withprimarily a higher lithia content than the glass-ceramic of the mainbody portion. When the surface layer is a glass, the glass-ceramic ofthe main body may have an average linear coefficient of expansionsomewhat less than, but preferably greater than, that of a thermallycrystallizable glass of the same overall composition so that due to thehigher lithia content of the glass layer in that article of theinvention the glass has an average coefficient of linear expansionsubstantially less than that of the glass-ceramic main body portion.

In one embodiment of the process of the invention, the composition ofthe thermally crystallizable glass, used to form the initial article,contains a concentration of an oxide of an alkali metal that inhibitsthe rate of bulk crystallization so that for a particular heat treatmentthe rate of such crystallization is substantially less than that of acomposition differing only by the absence of that alkali metal oxide.This inhibition can be enhanced by using a composition in which aningredient that combines with alumina and silica during bulk in situcrystallization is present in the low portion of its required range ofconcentration in the glass for the latter to be a thermallycrystallizable glass. The nucleant content in the glass is Within therange required for bulk crystallization and is preferably above theminimum portion of the range. As a result, the glass article can beheated to an elevated temperature, e.g., 600 C. to 700 C., which issubstantially below its softening point, to obtain surfacecrystallization which provides a surface layer that does not soften whenthe article is then heat treated at higher temperatures for nucleationand finally still higher temperatures for bulk crystallization of themain body portion of the article. Of course, the temperature used forsurface crystallization, which does not require nucleation of thenucleant in the surface layer, may be at the temperature that providessuch nucleation in the main body portion. In any event, the surfacelayer of glass-ceramic obtained by surface crystallization minimizessagging or other shape change during the bulk crystallization.

This latter embodiment of the process of the invention is useful in theother embodiments in which the article of glass-ceramic obtained by thebulk crystallization is further treated by additional steps includingion exchange, but is also useful in making that glass-ceramic articleitself. To accelerate the rate of surface crystallization, the originalglass article may be ion exchanged when another alkali metal oxide isone of the ingredients that participates to form a crystalline phase. Insuch case, an ion exchange step is performed on the glass to replace inits surface layer at least part of the inhibiting alkali metal ion bythe participating alkali metal ion. For example, in a glass that formsby crystallization glass-ceramic in which the crystalline phase containsbetaeucryptite or beta-spodumene or both, sodium is the inhibiting ionand is replaced by lithium in a surface layer prior to the surfacecrystallization. In the event the foregoing process is followed byconversion of the surface layer to glass with subsequent crystallizationof the surface layer, ion exchange of the glass surface layer is notperformed prior to the crystallization of it to a glassceramic having adifferent crystalline phase than that of the main body portion, unless athicker surface layer is desired.

BACKGROUND OF THE INVENTION Glasses that are controllably crystallizableby a heat treatment are commonly referred to as thermally crystallizableglass compositions. The glass-ceramics are the products obtained fromthese controllably crystallizable inorganic glasses by a suitable heattreatment, and glassceramics are also referred to as thermallycrystallized glasses. Thus the term noncrystalline glass excludesglass-ceramics but for convenience the termglass is used in thisapplication to provide such exclusion and. therefore, to meannoncrystalline glass.

There are many types of silicate glasses that are thermallycrystallizable glass compositions. A glass-ceramic body contains manysmall crystals in a glass matrix. The crystalline phase ofglass-ceramics can contain one or more crystalline materials. Thecrystalline materials that are formed depend upon the originalcomposition of the thermally crystallizable glass and often depend uponthe nature of the heat treatment.

The expansion coefficient of thermally crystallizable glass is dependentupon the glass composition. There can be a substantial differencebetween the expansion coeflicients of thermally crystallizable glassesthat are not members of the same glass system. Also, the expansioncoefficients of the glass-ceramics can differ greatly. The actualexpansion coefficient of a glass-ceramic depends on the compositionalingredients and on the temperatures and times of the heat treatment usedto form the glassceramic from the thermally crystallizable glass.

Articles of glass-ceramic material are made by melting batch ingredientsto provide molten thermally crystallizable glass and thereafter formingfrom the molten glass by conventional means, such as press molding,casting, blow molding, and tube and rod drawing, useful glass articles.One type of useful article is tableware such as plates, cups, and teapots. Tableware is usually made by pressing in a mold or by blow moldingtechniques. The articles of thermally crystallizable glass are subjectedto a controlled heat treatment to convert the glass to a glass-ceramic.

Some glass-ceramics are compositions that contain one or more alkalimetals, expressed as oxide as part of an overall composition alsoexpressed primarily as oxides. Many of the thermally crystallizableglass compositions are of the lithia-alumina-silica system containing aminor amount of at least one nucleating agent for the glass, such asZrO; "H0 and SnQ By controlled in situ crystallization there is obtainedglass-ceramic that contains in a glass matrix predominantlylithia-containing crystalline phases, either beta-eucryptite orbeta-eucryptite-like crystals or beta-spodumene or beta-spodumene-likecrystals, or both, as indicated by X-ray diffraction data.

Copending US. Patent application Ser. No. 352,958, filed on Mar. 18,1964, by William E. Smith and entitled Glasses, Ceramics and Method,with a common assignee, now US. 3,380,818, discloses and claims anotherclass of glasses and glass-ceramics that comprise silica, alumina,lithia, magnesia and a limited amount of both zirconia and titania. Thatapplication is hereby incorporated by reference.

US. application Ser. No. 352,958 describes the manner of heat treatmentto convert the crystallizable glass composition to glass-ceramic. Themaximum temperature reached in the heat treatment ranges from about 1400F. to 2100 F. and the period of time at the final temperature used isdependent upon the degree of crystallization desired in the product andupon the actual maximum temperature. When the maximum temperature islimited to the range of about 1400 F. to about 1675 F. it is indicatedin that patent application that in the crystalline phase that is formedbeta-eucryptite or betaeucryptite-like crystals predominate. When thefinal or maximum temperature of the heat treatment is above about 1650F. the crystals formed constitute a mixture of beta-eucryptite andbeta-eucryptite-like crystals and beta-spodumene and beta-spodumene-likecrystals. At the maximum heat treatment temperatures of about .1800 F.to 2100 F. the crystalline phase is primarily betaspodumene crystals andbeta-spodumene-like crystals.

The glass-ceramic of that patent application has an expansioncoefficient that is dependent upon. the final temperature of the heattreatment. When the glass-ceramic results from a final heat treatmenttemperature of a maximum of about 1675 F., the expansion coeflicient issubstantially lower than when a higher final temperature for the heattreatment is used. In other words, the glass ceramic, in which thecrystalline phase can be considered as being beta-eucryptite, has asubstantially lower expansion coefficient than the glass-ceramic of thesame composition having a crystalline phase that can be considered asbeing beta-spodumene.

Copending US. Patent application Ser. No. 464,147 filed by Clarence L.Babcock, Robert A. Busdiecker and Erwin C. Hagedorn on June 15, 1965,and entitled Prodnot and Process for Forming Same with common assigneenow abandoned discloses and claims a further class of thermallycrystallizable glass compositions and glass-ceramics made from theseglasses. That application is hereby incorporated by reference.

The Babcock et al. patent application Ser. No. 464,147 discloses thediscovery that a crystallizable glass composition, containing thefollowing essential components, present in the glass composition in thefollowing weight percent limits, can be treated at a finishingtemperature that 5 can be varied at the maximum within about 50 to 100F. or more, without affecting the substantially uniform, low expansioncharacteristics imparted to the transparent glass-ceramic which isformed, the glass-ceramic having a coefficient of linear expansion ofabout 10 10" to about 10 10- per C. over the range to 300 C.

where the ratio of (CaO-l-MgO+ZnO+Na O-{-B O to Li O is less than 2.4and the ratio of SiO to A1 0 is no more than 3.8 and is usually no morethan 3.3. An example of this class of thermally crystallizable glassesthat provides a glass-ceramic having an average coefficient of linearexpansion of 0X 10 C. (0-300 C.) has the following theoreticalcomposition and for an actual tank batch had the following analyzedcomposition, expressed as various oxides and one chemical element inweight percent:

Percent Theoretical Analyzed 1 Not analyzed.

The differences are believed to be due to alumina pickup andvolatilization loss in the case of ZnO.

This glass-ceramic as an article was prepared by placing the glass as anarticle about 2 inches thick and at 1300- 1700 C. in a preheated oven at1000 F. The oven temperature was increased to 1150 F. because of thepresence of the hotter glass article and oven was then maintained at1150 F. for 3 hrs. followed by increasing the oven temperature to 1350"F. at a rate of 5 F./minute and then maintained at 1350 F. for 50 hours.The glass article then was cooled at the rate of 1 F./minute until 1000F. was reached, then 5 F./minute until room temperature was reached.

Copending U.S. Patent application Ser. No. 362,481 filed by Robert R.Denman on Apr. 24, 1964, and entitled Ceramics and Method with commonassignee now U.S. 3,428,513 describes a process of improving the modulusof rupture of certain compositions of glass-ceramic by an ion-exchangeprocess in which lithium ions in a surface layer of the glass-ceramicarticle are replaced by larger alkali metal ions, specifically sodium orpotassium ions. The glass-ceramic is in the form in which thecrystalline phase is primarily beta-spodumene crystals andbetaspodumene-like crystals. To date none of the ion-exchange materialsused has provided a similar result with a strength increase for a.glass-ceramic of such certain compositions, that are specified in saidapplication Ser. No. 464,147 and in the next paragraph in which thecrystalline phase is primarily beta-eucryptite or beta-eucryptite-likecrystals. That Denman patent application is also incorporated byreference. The thermally crystallizable glass composition that forms theglass-ceramic used has the following weight percentage limits based onthe total composition:

In addition, small amounts of residual arsenic and antimony oxides areoften present in the composition, since arsenic or antimony compoundsare often used as fining or oxidizing agents. In actual practice,arsenic, expressed as As O is usually present in amounts not more than0.3 weight percent, and antimony, expressed as Sb O is seldom present inamounts over 1 weight percent. Sodium oxide is often present in theglass to a certain degree from the raw materials, usually in amounts notover 1.5 or 2 weight percent. Further, when AS203 is used as a finingagent, it is commonly added together with a little NaNO a well-knownpractice. Another additive sometimes employed isF, usually in amountsnot exceeding 0.3 weight percent. It is added as a salt in the usualmanner and seems to aid the crystallization process somewhat when it isemployed. Thus, it seems to accelerate the rate of crystallization,sometimes to such an extent that harmful exothermic effects result;hence, it is usually undesirable to have any more than 0.25 to 0.3fluorine present, expressed as weight percent F, in the final glasscomposition.

This glass is formed, e.g., by melting a batch of suitable ingredientsin a gas-fired furnace at a temperature of about 2900 F. and afterfining is cooled to a suitable temperature for flowing, casting or anyother feeding step to form glass articles which are then heat treated,first, at a low temperature to form many nuclei or crystallites, andthereafter at a higher temperature to complete crystallization to thedesired degree. The final maximum crystallization temperature is about1800 F. to 2100 F. and the average coefiicient of linear expansion(0-300 C.) is less than 20 10 C. and usually about 15 10-' C. In oneexample a glass-ceramic article having, and made from a thermallycrystallizable glass having, the following analyzed composition wasimmersed in a molten bath of sodium nitrate at 750 F. for one-half hourfor some articles and for 3 hours for others, followed by cooling, waterwashing and drying:

Ingredient: Weight percent S102 70.4 A1 0 16.8 MgO 4 3.5 Zro 1.3 Ti0 1.8P 0 1.5 F 0.09

N820 As O 0.15

The original glass had an annealing point of about 1220 F. Theion-exchanged glass-ceramic articles had very high unahraded and highabraded values of modulus of rupture.

The preceding paragraphs refer to ion exchange for the purpose ofimproving the strength of a specific type of glass-ceramic. Before thatinvention the prior art had disclosed the technique of changing theproperties of glass articles by ion exchanging one alkali metal foranother in the surface layer of the glass article. This ionexchangeprocess can be one of two types of substitution. In the one embodimentthe replacing alkali metal ion has a larger ionic diameter or radiusthan the alkali metal ion being replaced. In the second embodiment thereplacing alkali metal ion has a smaller ionic diameter than that of thealkali metal ion being replaced.

H. G. Fischer and A. W. LaDue disclose and claim in their copending U.S.Patent application Serial No. 504,- 159, filed on October 23, 1965, andentitled Process and Product with common assignee, a method in which ionexchange of one alkali metal for another is accomplished by using aliquid medium containing an alkali metal salt of an organic acid.

E. F. Grubb and A. W. LaDue in their copending U.S. Patent applicationSerial No. 529,215, filedon Feb. 23, 1966, and entitled Process andProduct" with common assignee, now U.S. 3,498,773, discloses and claimsanother ion-exchange method in which the alkali metal ion, that is tosubstitute for another alkali metal ion in the surface layer of theglass article, is used as a compound that is not molten when in contactwith the glass at the elevated temperature used for the ion exchange.

In view of the methods of said Fischer et a1. and Grubb et al., herebyincorporated by reference, it will be apparent that there have now beendeveloped several diflerent techniques for ion exchange.

The ion-exchange process has been used to treat glass, that is notthermally crystallizable, to convert a surface portion of the glassarticle to a composition that at the temperature used for the ionexchange, if sufliciently high, will crystallize to form a glass-ceramicin which the crystals are referred to as beta-spodumene crystals. If theentire article were of this surface layer composition, it is reasonableto expect for some specific compositions that only the surface wouldcrystallize. The product of this process is referred to as a surfacecrystallized glass article, as distinguished from the conventionalglassceramic which is commonly referred to as bulk crystallized product.As a result of this process in which sodium ions are replaced by lithiumions of a molten lithium salt bath in which the glass article isimmersed at the elevated temperature, the compositional change is suchthat surface crystallization occurs while the main body or interiorportion remains unchanged in composition and thus remains as glass. Thisprocess is disclosed in U.S. Pat. No. 2,779,136.

Copending U.S. Patent application Ser. No. 371,089, filed on May 28,1964, by William E. Smith and entitled Glass, Ceramics and Method withcommon assignee, now U.S. Pat. 3,528,828 discloses and claims a type ofthermally crystallizable glass that, for example, as a glass has anexpansion coefficient of about 90X 10 C. but as a glass-ceramic has anexpansion coefficient between 100 10-"/ C. and 120 lO-' C. For theglass-ceramic the actual value of the coefficient is determined by thetemperature and time of heat treatment for the controlledcrystallization. The crystalline phase of that glass-ceramic isnepheline. That application is hereby incorporated by reference for aperiod of time, generally minutes to 6 hours.

Another type of composition of thermally crystallizable glass and theglass-ceramic made from it are disclosed in British specification No.869,328. The ingredients include silica, alumina and soda and thus theglass-ceramic has nepheline as its primary crystaline phase.

William E. Smith in another copending U.S. patent application, which isapplication Serial No. 5 32,058, filed on March 7, 1966, and entitledProcess and Product with common assignee, now U.S. Pat. 3,528,828discloses and claims a process for treating an article of aglass-ceramic to convert at least an area of its surface layer to anoncrystalline glass under conditions to maintain the main body portionof the article as a glass-ceramic. That invention requires that theglass-ceramic has a coefficient of linear expansion that is at least 5%and is a maximum of about 200% greater than that of the noncrystallineglass of the same overall chemical composition, which, of course, is theglass formed as a surface layer by the process. That application ishereby incorporated by reference.

OBJECTS OF THE INVENTION It is an object of the present invention toprovide an article with a main body portion that is a glass-ceramic andat least one surface layer that is either a glass-ceramic or a glass, inWhich the surface layer (1) has a lower coefficient of linear expansionthan that of the main body portion, (2) has a compressive stress Whereasthe main body portion has a tensile stress, and (3) differs (a) byoverall chemical composition from that of the main body portion, (b) bygreater crystal concentration or by the predominant crystal of thecrystalline phase of the glassceramic from the crystal concentration orfrom the predominant crystal, respectively, in the crystalline phase ofthe glass-ceramic of the main body portion, or (c) by its glass matrixhaving a different chemical composition, e.g., higher lithia content,than that of the glass matrix of the main body portion.

It is another object of this invention to provide processes for themanufacture of such article.

It is still a further object of the invention to provide a process ofmanufacturing a glass-ceramic article by in situ crystallization of aglass article by a process that minimizes change in shape of the articleduring the controlled heat treatment to convert the glass to aglassceramic, and especially to use this process as part of an overallprocess to make an article that is of the type recited above as thefirst object of the present invention.

These and other objects of this invention will be apparent to oneskilled in this art from a description of various embodiments of theinvention that follow.

DESCRIPTION OF THE INVENTION The following examples are presented forthe purpose of illustrating various embodiments of the process of thepresent invention and, of course, the products obtained by theseembodiments of the process are illustrative of the product of thisinvention. The first example is a description of one embodiment, alongwith details of illustrative conditions used in carrying out the processand an evaluation of the product obtained from the standpoint of itsflexural strength, i.e., modulus of rupture, without abrasion of theproduct and after abrasion. For comparison there are presented theunabraded and abraded flexural strengths of untreated glass-ceramicobtained by the in situ crystallization but without the treatment forion exchange. The temperature used for the ion exchange will result inan in situ crystallization of a suitable glass for the production ofbetaeucryptite crystals as a crystalline phase.

The flexural strength or modulus of rupture of a glass or aglass-ceramic is determined in the following manner. Glass cane isobtained by pulling it from molten glass. The glass cane is cut into5-inch long sample rods that have a diameter of about inch.

When the flexural strength is to be determined for glass-ceramicobtained from such glass, as is the present case, the glass cane isconverted to a glass-ceramic by a heat treatment. First the cane isheated to and maintained at a suitable temperature for the formation inthe glass of crysals of a nucleant, such as titania or titania andZirconia, followed by a predetermined pattern of heat treatment athigher temperatures to obtain a crystalline phase from the maincomponents of the glass. The balance of the glass ingredients remain asa glass matrix. The resultant glass-ceramic sample rods are then testedfor flexural strength after or Without abrasion. In the present case theabrasion comprised manually rubbing the sample rods of glass-ceramicWith No. 320 emery paper.

The flexural strengths of the sample rods are determined using aTinius-Olson testing machine. This machine applies a measured loadthrough a single knife edge to the center of the sample rod supported ontwo knife edges that are four inches apart (3-point loading). The loadis applied at a constant rate of 24 lbs. per min. until failure occurswith a marker indicating the highest load applied to the point offailure. A dial micrometer calibrated in inches and equipped with a barcontact instead of a point contact was used to measure the maximum andminimum diameters at the center of the sample to an accuray of 0.0005inch. Since few sample rods are perfectly round. the load is appliednormal to the maximum diameter and the standard formula for anelliptical cross-section is used in calculating the modulus of rupture(MR) as follows:

The modulus of rupture in this formula gives the flexural strength inpounds per square inch of cross-sectional area at failure. The data forflexural strengths are based on the average of the values obtained for anumber of sample rods.

EXAMPLE I A molten glass was made with batch materials to provide thefollowing composition:

Ingredients: Weight percent SiO 47.2 A1 31.1 MgO 10.5 Na O 1.5 Zro 8.0Tio 1.5 SnO 0.2

This glass was made in a pot in a conventional manner that is well knownin the art. This glass had a liquidus of greater than 2760 F. Glass canewas drawn from the molten glass. The silica, alumina and magnesiacontents were within the broad and preferred ranges of glasses that areutilized in the process disclosed and claimed in US. Pat. No. 3,117,881.The glass contained, as nucleants, titania and a larger amount ofzirconia. These are utilized in various compositions of that patent. Thetin oxide would provide the function in the glass manufacture that isdescribed in that patent. The 1.5% by weight of soda in the glass waswithin the broad and preferred ranges of modifying agents permitted inthe thermally crystallizable glass of that patent. This specific glasshad an average coefficient of linear expansion (0300 C.) of 33.7 C.

The glass cane was cut into sample rods. Most of the sample rods wereheat treated as follows to convert them to a glass-ceramic. These rodswere heated to and maintained at 1450 F. (788 C.) for 1 /2 hours, heatedto and maintained at 1750 F. (954 C.) for 1 /2 hours, heated to andmaintained at 2000 F. (1093 C.) for 1%. hours, and then cooled graduallyto room temperature. By this treatment the glass of the rods wasconverted to a glass-ceramic. Cordierite crystals are the predominantcrystals in the crystalline phase. This glass-ceramic had an averagecoefficient of linear expansion (0300 C.) of 59.0 l0- C. which isgreater than that of the glass.

Sample rods of this glass-ceramic after preheating for one-quarter hourat 1450 F. while suspended in a tubular furnace were moved laterally bymoving the tubular furnace directly above another furnace containingsalt in a crucible liner in a metal container. The latter furnace wasmaintained at a temperature of 1450 or 1500 F., at which temperaturesalt was molten. The molten salt bath contained, on a weight basis, 75%lithium sulfate, 24% potassium sulfate and 1% sodium hydrogen sulfate.The rods were lowered for immersion in the molten salt for specificperiods of time, raised from the bath, cooled gradually in air, waterwashed and then dried.

Most of the ion-exchanged treated sample rods of glassceramic wereabraded, as described above. Some were tested for flexural strengthwithout any abrasion. Sample rods of glass-ceramic that had not beensubjected to the molten salt bath immersion were also tested forflexural strength, some without abrasion and some with abrasion. Theresults are tabulated below:

Time of Flexure strength, p.s.i. Temperature of salt bath, immersion, F.hrs. Unabraded Abraded As seen in some of the copending patentapplications mentioned above, a temperature of 1450 and 1500 P. willprovide an in situ crystallization to form a crystalline phase thatcontains predominantly beta-eucryptite. Thus the temperature of themolten salt treatment was sufficiently elevated to provide in thatexterior part at least of the surface layer an in situ crystallizationof part of the glass matrix of the initial glass-ceramic changed incomposition by replacement of sodium ions by lithium ions. Thecoefficient of linear expansion of 59 10- C. of the initialglass-ceramic would certainly be greater than that in the surface layerafter substitution of lithium for sodium in the glass matrix andespecially after conversion of part of the changed glass matrix in atleast the exterior part of the surface layer to beta-eucryptite in viewof its very low expansion coefiicient as compared with cordierite. Thusthe process provides in at least the exterior part of the surface layerby ion exchange, a reduction in the expansion coefficient. This canaccount for the improved flexural strength after the process of ionexchange under conditions that provide also in situ crystallization.This compressive stress surface layer also provides a retention during aspecific abrasion of flexural strength greater than that of theunabraded initial glass-ceramic.

For comparison, sample rods of the initial glass were similarlypreheated for one hour at 1400 F. or 1450 F., were immersed for one-halfhour in that molten salt at the temperature of preheating and thentreated further. In the case of the glass rods that were immersed at1400 F., they were slowly cooled in air at the conclusion of the saltimmersion. An examination of these treated glass rods indicated that nocompressive stress surface layer had been formed. In the case of theglass rods immersed in the molten salt at 1450 F., they were posttreated after the immersion in the molten salt by heating for one hourat 1720 F. (938 C.) and then for another 1 /2 hours at 1920 F. (10'49C.) which resulted in rods that had wrinkled surfaces. It was concludedthat these treatments of glass rods resulted in relatively weak rods.These results in comparison with those using the ion-exchange salttreatment of the glass-ceramic rods show that, with this type of overallcomposition, the improvement in strength is obtainable only when anarticle is treated with molten salt at such temperature when thiscomposition is a glassceramic rather than a thermally crystallizableglass.

Pure stoichiometric cordierite is a magnesium aluminosilicatecrystalline material with the formula The glass composition of thisexample by the in situ crystallization produces a cordierite crystallinephase and a glass matrix in which the magnesia content and the sodacontent are substantially less than and substantially greater,respectively, than that of the initial glass. The' replacement of sodiumions by lithium ions in the surface layer of such glass matrix providesa lithia content for the glass matrix such that an in situcrystallization of betaeucryptite occurs at the ion-exchange treatmentif sufliciently high or if not by later heat treatment. This lowersadditionally the expansion coeflicient of the surface layer.

The interior portion of the surface layer may not have sufiicient lithiacontent to provide crystallization, but it will lower somewhat theexpansion coefficient of that portion of the surface layer.

If a temperature sufficiently lower than that used in Example I for theion exchange is used, the time of treatment would be increased for acomparable degree of ion exchange. Such lower temperature can besufficiently low that in situ crystallization will not occur. Then theglassceramic after such ion exchange with suitable lithiumcontainingmedium is followed by a heat treatment of the rods at an elevatedtemperature sufficient to provide the in situ crystallization, therebyattaining generally the same result as described above for theion-exchange treatment at an in situ crystallization temperature.

In view of the foregoing presentation of prior art and Example I for oneembodiment of the present invention, it is not necessary to presentother embodiments of the invention in the same amount of detail. Theseembodiments can be utilized by one of ordinary skill in this art in viewof this entire disclosure and the prior art.

The embodiment that is illustrated by Example I is the process ofconverting an article of thermally crystallizable glass composition to aglass-ceramic followed by replacement of an alkali metal ion of thesurface layer of the glass matrix by an alkali metal ion having asmaller ionic radius at a temperature that provides in situcrystallization or at a lower temperature followed by heating at atemperature for such crystallization in at least the exterior portion ofthe surface layer wherein the formed crystals are different than thosein the crystalline phase of the initial glass-ceramic and have a lowercoefiicient of expansion. Of course, in this example the surface layerwill contain, in addition to the newly formed crystals, the crystals ofthe main body portion of the glass-ceramic rods but these newly formedcrystals are not necessarily separate from the initial crystallinephase. They may be formed on the surfaces of the initial crystallinematerial and, of course, formation of the new crystalline material asseparate crystals is not a necessary part of the present invention.Furthermore in certain cases ion exchange without concomitant orsubsequent in situ crystallization is one embodiment of the presentinvention, whether the surface layer is glass or glass-ceramic. Ofcourse, the main body portion is glass-ceramic.

EXAMPLE II An article of thermally crystallizable glass composition ofthe type, that by in situ crystallization forms a glassceramic with acrystalline phase that is predominantly nepheline, is converted to anarticle of glass-ceramic by suitable heat treatment, as describedearlier with reference to British Pat. No. 869,329 and Smith patentapplications Ser. Nos. 371,089 and 532,058.

In one embodiment the heat treatment at a specific final temperature isfor a period of time less than one half of that which will produce themaximum amount of crystalline phase so that the glass matrix of theglassceramic will contain a substantial amount of soda. In thisembodiment the glass-ceramic article is treated with alithium-containing ion-exchange material, such as the molten mixture ofsalts including lithium sulfate mentioned above, at an elevatedtemperature that is at least about 200 C. (about 400 F.), and preferablyat least about 350 C. (about 660 F.), for a period of time to replacesodium ions by lithium ions in the glass matrix in a surface layer ofthe glass-ceramic article. This ion exchange converts the glass matrixin the surface layer to a glass composition having a lower expansioncoefficient than the initial glass matrix and thus lower than that ofthe glass matrix in the main body portion of the glassceramic article,so that the surface layer has a compressive stress and the main bodyportion has a tensile stress. When the ion-exchange treatment isconducted at an elevated temperature that is below the strain point ofthe glass matrix, it is necessary to heat the article after the ionexchange to a higher temperature sufficient to relieve tensile stress inthe surface layer created by the smaller ions replacing the larger ions.The compressive stress in the surface layer is created because of thedifference in expansion coefficient between that layer and the main bodyportion. It is preferred that the ion exchange be conducted at atemperature above the strain point of the glass matrix of the initialglass-ceramic article. A tem perature between 500 C. (about 950 F.) and850 C. (about 1560 F.) is preferred. A temperature of at least 700 C.(about 1300 F.) is especially preferred. When the time and temperatureof the ion exchange are sufficiently long and high, respectively, andwith suitable choice of initial glass composition and of extent ofcrystallization prior to the ion exchange, there is a further in situcrystallization in the surface layer. The new crystals arebeta-eucryptite. The surface layer already contains a crystalline phasewhich, of course, is predominantly nepheline. This additionalcrystallization in a surface layer results in a different glass-ceramiccomposition than that of the main body portion. Beta-eucryptite has alower expansion coefficient than nepheline. Thus the glassceramic of thesurface layer has a lower expansion coefficient than the main bodyportion. Also this glassceramic of the surface layer has a lowerexpansion coefficient than that of the surface layer in which only ionexchange occurs.

In another embodiment using this glass-ceramic in which nepheline is thepredominant crystal in the crystalline phase, the surface layer of theglass-ceramic article is heat treated, such as by flame treating, inaccordance with the process disclosed in Smith Patent application Ser.No. 532,058, and ion-exchange treated to replace sodium ions in thesurface layer of thermally crystallizable glass by lithium ions in themanner mentioned above. The glass of the surface layer before theion-exchange treatment has a lower expansion coeflicient than theglassceramic main body portion. This difference is further enhanced bythis ion exchange. By judicious choice of initial glass composition anddegree of ion exchange, part of the glass surface layer can be convertedto crystals of beta-eucryptite with heat treatment at temperaturesdescribed earlier, because such compositions contain substantial amountsof nucleant and substantial contents of the three ingredients that formbeta-eucryptite.

The process of the Smith application Ser. No. 532,058 excludes the useof a glass-ceramic that has a lower coefiicient of linear expansion thanthe thermally crystallizable glass from which it is obtained. Theconversion of a surface layer of an article of such glass-ceramic to theglass followed by cooling the glass layer results in spalling orbreaking away of the surface layer because a tensile stress is createdin the surface layer by this difference in expansion coefficient.

If there is an ion exchange of a larger ion for a smaller ion afterforming the glass layer as in one embodiment of that Smith process, thecooling is only to a temperature below the annealing point and usuallybelow the strain point of the glass at which the exchange takes place toprovide a compressive stress. However, cooling to such temperature alsocreates this undesirable tensile stress.

The second embodiment of the present invention avoids such spalling,because the ion-exchange process of such glass surface layer can becarried out at a temperature above the annealing point. A subsequentcooling of the article to room temperature without damage by spalling ispossible, because the surface layer now is a glass that has a lowerexpansion coefficient than that of the glassceramic main body portion.When the process of this invention converts the ion-exchanged glasssurface layer to a glass-ceramic layer during or after the ion exchange,the article is ultimately cooled to room temperature. There is nospalling damage, because the glass-ceramic of the 13 surface layer has asubstantially lower expansion coefficient than that of the glass-ceramicof the main body portion and thus a compressive stress has been createdin the surface layer.

The foregoing advantage of this embodiment, as compared with thelimitation of the process of Smith application Ser. No. 532,058 isbecause an alkali metal ion is replaced by another alkali metal ionhaving a smaller ionic radius. This replacement is performed at or isfollowed by heating the article to a temperature above the strain point,even above the annealing point.

EXAMPLE III This example uses an article of a thermally crystallizableglass composition such as disclosed in Table II of US. Pat. No.2,920,971 or US. Patent applications Ser. Nos. 352,958, 362,481 and464,147.

In one embodiment, thermally crystallizable glass of the article isconverted to a glass-ceramic in which the crystalline phase ispredominantly beta-spodumene and beta-spodumene-like crystals so thatthe article of glassceramic formed has an expansion coeificient between10X 10* C. to 20 X l- C.,

such as 15 10-"/ C. The surface layer of this glassceramic article isheated to convert it, without converting the main body portion, back tothe thermally crystallizable glass. This glass is converted by heattreatment, under conventional conditions of the type described earlierin connection with the prior art, to convert it to a glass-ceramic inwhich the crystalline phase is predominantly beta-eucryptite. Theresultant article has a surface layer of a different glass-ceramic witha lower coefficient of linear expansion than that of the glass-ceramicof the main body portion. Based upon the thermal treatments received bythe glass of the surface layer and the glass of the main body portion,it is expected that the surface layer has an expansion coefficient, forexample, of

while the main body portion has an expansion coefficient of x10 C. Inthis case, there is no difference in overall chemical compositionbetween the surface layer and the main body portion. The difference isin the nature of the crystals of the crystalline phase of the twoglass-ceramics that constitute the surface layer and the main bodyportion.

In the foregoing first embodiment of this example, the initial thermallycrystallizable glass has a higher expansion coefficient than that of theglass-ceramic of that composition and containing beta-spodumene. Theconversion of the surface layer of the article from this glassceramic tothe thermally crystallizable glass results initially in a product havinga surface layer with a higher expansion coefficient than that of themain body portion. In this embodiment, it is desirable, and when thedifference is too great, is necessary that the article is maintainedabove the strain point and preferably above the annealing point betweenthe heat treatment that forms the glass surface layer and the heattreatment that converts the glass of the surface layer to aglass-ceramic in which beta-eucryptite is the predominant crystallinematerial. Accordingly, the entire article is preferably heated to asubstantial elevated temperature, preferably above the annealing pointof the thermally crystallizable glass, before the heat treatment, suchas flame treatment, of a surface layer of the glass-ceramic to form theglass surface layer and the entire body is maintained at suchtemperature until in situ crystallization occurs in the surface layer.Of course, this temperature is below the temperature at whichcrystallization occurs to form betaspodumene. This is the preferredembodiment of the present invention.

Under certain conditions including the maximum temperature reached bythe surface layer and dependent upon the titania content of the initialthermally crystallizable glass, titania may be redissolved when thesurface layer is heated to form a surface layer of glass. In such event,the temperature of the entire article is. maintained such that thetemperature of the newly formed glass surface layer is lowered to thenucleation temperature. The temperature of the article is then raised tocause the in situ crystallization for beta-eucryptite crystal formationby a programmed heat treatment as described in the US. Patent and patentapplications mentioned above.

In a second embodiment of this example, the article is made of thermallycrystallizable glass that contains only a minimum content of lithia anda maximum amount of soda but an adequate content of nucleant to providebulk crystallization. The article is entirely converted to theglass-ceramic by controlled heat. treatment. The article is ionexchanged in its surface layer to replace sodium ions with lithium ions.The temperature during the ion exchange is perferably at the highelevated temperatures mentioned above. At lower exchange temperaturestensile stress is created, but the later in situ crystallizationtemperature relieves such stress and. creates compressive stress.However, while at the lower temperature damage to the article can occurin certain cases. The time and temperature for ion exchange can providefor an in situ crystallization of beta-eucryptite in the surface layerduring at least the final period of the ion-exchange treatment. Thisglass-ceramic obtained from the ion-exchanged surface layer has a lowerexpansion coefiicient than that of the main body portion. This aspect ofthis invention is thus (1) the difference in expansion coefficients ofthe glass matrix of the surface layer and the main body portion, (2)additional crystal formation With added lithia to form beta-eucryptiteeither during the ion exchange or later, (3) reduced soda content in theglass matrix of the surface layer and (4) reduced. molar content of allalkali metal in this glass matrix that results in a lower expansioncoeificient than the initial glass matrix.

The second embodiment is modified by use of a final temperaturesufficiently high to convert lithium values introduced by the ionexchange to beta-spodumene. This changes the composition of the glassmatrix to a composition having a lower expansion coefficient.Furthermore, the glass-ceramic of the surface layer has a lowercoefficient of expansion in this case because the concentration ofbeta-spodumene in the surface layer is greater than that in the mainbody portion. In this modification the surface layer, that is ionexchanged, is the initial glassceramic or the glass obtained from it.

In another embodiment of this example, the thermally crystallizableglass composition of the article may contain a small soda concentrationor content. It is preferred that the soda content be very low or thatsoda be absent. The article of this glass is ion exchanged to replacepart of the lithium ion content with sodium ions using one of thewell-known ion-exchange processes. The temperature is not required to bebelow the strain point for a long time of treatment or at a maximumtemperature of about F. above the annealing point for a short time,because this ion exchange is not performed to create a compressivestress surface layer. Thus the preferred temperature is substantiallyabove the annealing point, but below the softening point of the glass.

During the ion exchange of this embodiment the temperature may be suchthat nucleation may occur. The degree of nucleation may be insufficient.Thus the article after the ion exchange is subjected to heat treatmentfor nucleation. The glass article, now ion exchanged in a surface layerand nucleated throughout, is heat treated as indicated in the patent andapplications mentioned above to convert the main body portion to aglassceramic.

The extent of ion exchange is controlled for certain articles so thatthe composition of the surface layer is such that it will also formglass-ceramic, but at a slower rate than the crystallization of the mainbody portion. This is desirable to avoid flow of the glass surface layerat the high temperatures required for this in situ crystallization ofthe main body portion. Obviously, this is not a necessary limitationwhen only one surface of the article is ion exchanged and that surfaceis flat, because it can be maintained horizontal and above the main bodyportion to prevent gravity flow with peripheral guards, if necessary,during the heating for in situ crystallization.

After the article has its main body portion converted to theglass-ceramic, the article is then ion exchanged to replace sodium ionswith lithium ions in the entire surface layer, whether the surface layeris glass or glassceramic with its content, i.e., concentration, ofbetaspodumene being less than that in the main body portion. Then thearticle is thermally treated under the proper program of heat treatment,as disclosed in US. patent applications mentioned above, to convert theintroduced lithium and some ingredients including lithium alreadypresent to crystals of beta-eucryptite. In view of the replacement ofsoda content in the surface layer, this heat treatment by in situcrystallization to beta-eucryptite utilizes lithia content that did notform beta-spodumene wihin the time of the first in situ crystallization.Thus the surface layer is a glass-ceramic and now has a lower expansioncoefficient than that of the main body portion.

EXAMPLE IV The glass of the entire initial article is a thermallycrystallizable glass composition of the type of Example III. This glasscontains an amount of soda within the top portion of range of sodacontent that does not prevent in situ crystallization to formglass-ceramic that will contain either beta-eucryptite or beta-spodumeneor both. The glass article is treated by ion exchange to replace sodiumin its surface layer by lithium in accordance with any of the processesof ion exchange already known for glass, such as are described above.This results in a surface layer of thermally crystallizable glasscomposition that differs from that of the main body portion. The surfacelayer has a higher lithia content and a lower soda content than that ofthe main body portion. In this embodiment, the nucleating agent oragents are present at concentrations sulficient for bulk crystallizationonly at a slow rate, e.g., a rate less than one-fourth of the maximumrate obtainable with maximum permissible content of nucleant.

This article of glass with a surface layer of ion-exchanged glass isheat treated to a temperature that will provide a surfacecrystallization, i.e., an in situ crystallization in the surface layer,to form beta-eucryptite. This surface crystallization will occur beforein situ crystallization of the main body portion, because the surfacecrystallization is not dependent upon the presence of special nucleantsin the glass.

It is known in the prior art, as illustrated by the teaching of US. Pat.No. 2,779,136, that surface crystallization can occur at temperatures aslow as between 600 C. and 750 C. (between 1112 F. and 1382 F.). Thesetemperatures are below temperatures for bulk in situ crystallization.The advantage of this surface crystallization is that the subsequentprogram of heat treatment, including a nucleation heat treatment, iscarried out on an article that has a surface layer of glass-ceramicrather than glass. This surface layer of glass-ceramic avoids orminimizes change in shape due to sagging or flow during the bulkcrystallization temperature treatment.

This article, with its surface layer of glass-ceramic due to surfacecrystallization, is subjected to a program of heat treatment to convertthe entire article to a glassceramic that is predominantlybeta-eucryptite or betaeucryptite-like crystals or a mixture of theseand betaspodumene and beta-spodumene-like crystals or predominantlybeta-spodumene and beta-spodumene-like crystals. The PIOE S of thisexample uses a separate facet of the invention that does not require ionexchange and other steps subsequent to the bulk crystallizationtreatment.

EXAMPLE V The entire article obtained by Example IV is converted by heattreatment of prolonged duration to a glass-ceramic in whichbeta-spodumene and beta-spodumene-like crystals predominate in thecrystalline phase. This article of glass-ceramic containingbeta-spodumene is heat treated to form a surface layer of glass asdescribed in earlier examples. The glass is converted to beta-encryptitein accordance with one of the earlier embodiments of Example III.

EXAMPLE VI Example IV describes an aspect of the invention in whichthere is an ion exchange of an article of a thermally crystallizableglass to convert a surface layer to a composition that, 'by heattreating within a temperature range, will surface crystallize withoutbulk crystallization. The composition of the original glass will bulkcrystallize but at a slow rate due to its specific composition. Ionexchange changes this composition in the surface layer so the latterwill easily and rapidly surface crystallize. These thermallycrystallizable glasses form glass-ceramics in which alternativelybeta-eucryptite and beta-spodumene crystals or mixtures of both arepresent.

This advantage of surface crystallization of a glass prior to bulkcrystallization can be utilized with other types of thermallycrystallizable glasses, e.g., those that form a glass-ceramic in whichnepheline or cordierite are the predominant crystals. The glass is ionexchanged to replace sufficient sodium in the surface layer by lithium.During the ion-exchange treatment, if its temperature is sufficientlyhigh, or after its treatment at an elevated and suitable temperatureproduces surface crystallization. This article is heat treated, withless change of its shape, to a glass-ceramic for the main body portionthat is either a nepheline type, a cordierite type or some other typeother than beta-spodumene or beta-eucryptite. The glass-ceramic of thesurface layer is converted to a glassceramic in which the crystals arebeta-eucryptite or betaspodumene or both, dependent upon the finaltemperature required for in situ crystallization of the main bodyportion. That temperature depends on the type of initial glasscomposition.

The initial glass can be modified by incorporating a limited amount oflithia. This initial lithia content along with the lithia contentprovided by the ion exchange gives a surface layer composition that isconvertible to the glass-ceramic by surface crystallization as describedabove.

After the bulk crystallization, this article has a compressive stresssurface layer due to the difference in expansion coefficients betweenthe main body portion and the layer that are two differentglass-ceramics.

EXAMPLE VII The article obtained in Example V1 is selectively heated toconvert the surface layer to glass, while the main body remains asglass-ceramic followed by heat-treatment to glass-ceramic in whichbeta-eucryptite is the predominant crystal.

EXAMPLE VIII A glass article having the composition of Example VI ision-exchanged, surface crystallized and then bulk crystallized. Thesurface layer is then ion exchanged to substitute sodium for lithium inthe glass matrix that was not utilized in the surface crystallization toform, by further heat treatment, additional crystals comparable to thosein the main body portion. Thus the second ion exchange replaces lithiumby sodium.

EXAMPLE IX The surface layer of the article of Example VI after the bulkcrystallization is selectively heat treated, such as by flame treating,to form a glass, while the main body remains as glass-ceramic. Thisg'ass is ion exchanged to replace lithium with sodium followed by insitu crystallization at the surface layer so that the surface layer andthe main body portion now have the same overall composition. In thisalternative, the surface layer is used only for surface crystallizationto minimize shape change during subsequent bulk crystallization. It isnot used for the objective of making a final article with a surfacelayer having a compressive stress due to a lower expansion coefficientfor the surface layer as compared with a different overall compositionthan that of the main body portion.

EXAMPLE X An article of a glass containing on a weight basis thefollowing ingredients: 62.9% SiO 14% A1 6% MgO; CaO; 1.7% Li O; 4.3% TiO3.1% Na O and 3% E 0 is made. This glass has an expansion coefiicientbetween C. and 300 C. of X l0" C. This glass is Example 1 of Table 1 ofUS. Patent application Ser. No. 410,016 entitled Glass, Ceramic andMethoc filed by Richard W. Petticrew on November 9, 1.964, now US. Pat.3,540,893, with common assignee, and hereby incorporated by reference.

By heat treatment for two hours at 1300 F., two hours at 1450 F. and onehour at 1550 F., it is converted to a glass-ceramic with an expansioncoefficient, for the same temperature range, of 43 10 C. and a modulusof rupture of 66,000 p.s.i. The high strength is attributed by Petticrewpartially to a formation of lithiumcontaining crystals in a largeramount in the surface layer than obtained in the main body portion ofthe article. In the main body portion the crystalline phase of theglasscerarnic is predominantly cordierite.

This article of thermally crystallizable glass is treated, in accordancewith the present invention, by an ion-exchanging material, such asdescribed earlier, to replace at least part of sodium ions by lithiumions in the surface layer of the article, thereby increasing the lithiacontent of the surface layer. In the present process, the article isthen heat treated as described by the Petticrew patent application forbulk crystallization. Because of the ion exchange a higher concentrationof lithium-containing crystals, e.g., beta-eucryptite is formed in thesurface layer to provide a lower expansion coefficient for the surfacelayer than that obtainable by the heat treatment without the ionexchange.

In another embodiment, the glass ceramic article obtained in Example 1of Petticrews patent application is heated to convert a surface layer tothermally crystallizable glass while maintaining the main body portionas glass-ceramic. This glass surface layer is ion exchanged to replacesodium with lithium. The heat treatment of the ion-exchanged glass layerto glass-ceramic containing beta-eucryptite or beta-spodumene is thenperformed, as described in earlier examples.

The foregoing description of compositions has mentioned variousingredients. These constitute at least 90% and preferably at least 95%,by weight of the compositions.

The depth of the surface layer of the article of the invention can bevaried widely, e.g., from 10 microns to 200 microns or more. When thissurface layer is formed by ion exchange, the depth obtained by thisexchange is desirably less when a later ion exchange is used as part ofthe overall process because the second ion exchange should replace theion first substituted with an ion of the type first replaced. To do thiseffectively a shallow layer is desired in the first exchange.

The difference in average coefiicient of linear expansion between thatof the surface layer and that of the main body can be varied widely. Theminimum numerical value for the difference is dependent upon thecoefficient of the main body portion. If it is 10 10 C., the coeflicientsurface layer is desirably no higher than about 1% 5 l0-' C., andpreferably no higher than about 0 10 C. If it is 15 10- 'C., that of thelayer is preferably no higher than 10 10- C. If the coefficient of themain body portion is about. 10"/ C., the layers coefficient ispreferably a maximum of 30 10- C. which is easily accomplished when. thelayer is a g1ass-ceramic containing beta-eucryptite, beta-spodumene orboth as the predominant crystal. When the coefiicient of the main bodyportion is about 110 10 C., that of the layer is preferably a maximum ofabout 80 10 C., and preferably at least in an outer part of the surfacelayer of below 5O 10 C.

Many variations of the invention in view of this disclosure will beobvious to one of ordinary skill in the art. The foregoing examples havebeen presented for the purpose of illustration of various embodiments ofthe process and the product of the present invention which is notlimited thereto, but only by the claim that follows.

I claim: 1. A process of making an article of glass-ceramic whichcomprises (1) forming an article from a thermally crystallizable glasscontaining at least one ingredient that is lithia that combines withalumina and silica to provide bulk crystallization by a heat treatment,a nucleant present in sufiicient concentration for the bulkcrystallization. and sodium oxide in a concentration that inhibits therate of said bulk crystallization without preventing surfacecrystallization of said glass, (2) heat treating the article of glass atan elevated temperature sutlicient to provide crystallization of asurface layer of the article while the main body portion remains aglass, and (3) heat treating the surface crystallized article at ahigher elevated temperature to convert the main body portion by bulkcrystallization to a glassceramic, said lithia being present in asufiicient concentration in the thermally crystallizable glass toprovide, in absence of said rate-inhibiting sodium oxide, bulkcrystallization uniformly throughout the article, and said surface layerhaving a lower coemcient of linear expansion than that of the main bodyportion of the article of glass-ceramic, and thereafter (4) heating asurface layer of the article to an elevated temperature suificientlyhigh to convert the glassceramic of the surface layer to glass whilemaintaining the main body of the article as glass-ceramic, (5)ion-exchanging the surface layer after its conversion to glass toreplace at least part of the sodium ions by lithium ions, and (6) heattreating the article to convert the glass of the surface layer toglass-ceramic containing betaeucryptite as predominant crystal wherebysaid surface layer has a lower expansion coefficient than that of themain body portion.

References Cited UNITED STATES PATENTS 3,486,963 12/1969 Smith (SS-303,485,647 12/1969 Harrington 30 3,113,009 12/1963 Brown et al. 65-332,420,971 1/ 1960 Stookey 106-39 3,282,770 11/1966 Stookey et al. 65303,253,975 5/1966 Olcott et a1 161-1 3,498,775 3/1970 Simmons 65--333,573,020 3/1971 Karstetter 65-30 3,464,807 9/ 1969 Pressau 65-30 S.LEON BASHORE, Primary Examiner K. M. SCHOR, Assistant Examiner US. Cl.X.R. 6533

1. A PROCESS OF MAKING AN ARTICLE OF GLASS-CERAMIC WHICH COMPRISES (1)FORMING AN ARTICLE FROM A THERMALLY CRYSTALLIZABLE GLASS CONTAINING ATLEAST ONE INGREDIENT THAT IS LITHIA THAT COMBINES WITH ALUMINA ANDSILICA TO PROVIDE BULK CRYSTALLIZATION BY A HEAT TREATMENT, A NUCLEANTPRESENT IN SUFFICIENT CONCENTRATION FOR THE BULK CRYSTALLIZATION, ANDSODIUM OXIDE IN A CONCENTRATION THAT INHIBITS THE RATE OF SAID BULKCRYSTALLIZATION WITHOUT PREVENTING SURFACE CRYSTALLIZATION OF SAIDGLASS, (2) HEAT TREATING THE ARTICLE OF GLASS AT AN ELEVATED TEMPERATURESUFFICIENT TO PROVIDE CRYSTALLIZATION OF A SURFACE LAYER OF THE ARTICLESWHILE THE MAIN BODY PORTION REMAINS A GLASS, AND (3) HEAT TREATING THESURFACE CRYSTALLIZED ARTICLE AT A HIGHER ELEVATED TEMPERATURE TO CONVERTTHE MAIN BODY PORTION BY BULK CRYSTALLIZATION TO A GLASSCERAMIC, SAIDLITHIA BEING PRESENT IN A SUFFICIENT CONCENTRATION IN THE THERMALLYCRYSTALLIZATION GLASS TO PROVIDE IN ABSENCE OF SAID RATE-INHIBITINGSODIUM OXIDE, BULK CRYSTALLIZATION UNIFORMLY THROUGHOUT THE ARTICLE, ANDSAID SURFACE LAYER HAVING A LOWER COEFFICIENT OF LINEAR EXPANSION THANTHAT OF THE MAIN BODY PORTION OF THE ARTICLE OF GLASS-CERAMIC, ANDTHEREAFTER (4) HEATING A SURFACE LAYER OF THE ARTICLE TO AN ELEVATEDTEMPERATURE SUFFICIENTLY HIGH TO CONVERT THE GLASSCERAMIC OF TH SURFACELAYER TO GLASS WHILE MAINTAINING THE MAIN BODY OF THE ARTICLE ASGLASS-CERAMIC, (5) ION-EXCHANGE THE SURFACE LAYER AFTER ITS CONVERSIONTO GLASS TO REPLACE AT LEAST PART OF THE SODIUM IONS BY LITHIUM IONS,AND (6) HEAT TREATING THE ARTICLE TO CONVERT THE GLASS OF THE SURFACELAYER TO GLASS-CERAMIC CONTAINING BETAEUCRYPTITE AS PREDOMINANT CRYSTALWHEREBY SAID SURFACE LAYER HAS A LOWER EXPANSION COEFFICIENT THAN THATOF THE MAIN BODY PORTION.